An estimate of population sizes of burrowing seabirds at the Diego Ramirez archipelago, Chile, using distance sampling and burrow-scoping |
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Authors: | Kieran Lawton Graham Robertson Roger Kirkwood José Valencia Roberto Schlatter David Smith |
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Institution: | 1. Australian Antarctic Division, 203 Channel Highway, Kingston, Tasmania, 7050, Australia 2. Phillip Island Nature Park, PO Box 97, Cowes, Victoria, 3922, Australia 3. Instituto Antártico Chileno, Plaza Mu?oz Gamero 1055, Punta Arenas, Chile 4. Instituto de Zoología, Facultad de Ciencas, Universidad Austral de Chile, Casilla, 567, Valdivia, Chile
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Abstract: | The Diego Ramirez Islands lie 60 nautical miles southwest of Cape Horn and are the breeding site for three species of burrowing
seabirds: blue petrels (Halobaena caerulea), common diving petrels (Pelecanoides urinatrix) and sooty shearwater (Puffinus griseus). Burrowing seabirds are highly vulnerable to predation by introduced vertebrate pests, and Diego Ramirez is an important
breeding site because it is one of a few remaining subantarctic island groups with no introduced predators. Diego Ramirez
is the only known breeding site for blue petrels in the southeast Pacific region, holding about 80% of the global population
of that species, and with a population ten times larger than any other population in the world. We estimated the population
size in 2002, using a novel application of the distance sampling technique to determine burrow density, and a burrow-scope
with excavations to determine occupying species. We found that density was correlated with slope angle and soil wetness. Burrow
densities in flatter terrain with drier soils were 2.03 burrows/m2 (95% confidence intervals: 1.82–2.27) and 1.11 burrows/m2 (0.84–1.48) in steeper terrain with wetter soils. The occupation rate of burrows were significantly different between habitat
types (t=2.74, d.f. 11, P<0.05); in flatter drier habitats the proportion of burrows that led to a nest was 0.85 (0.74–0.96), in steeper wetter habitats
this decreased to 0.64 (0.50–0.78). We used a digital elevation model to calculate true area rather than planar area for the
two habitat types on the main island of Bartolome, and charts to calculate planar area for the remainder of the archipelago.
There were 1.35 (1.15–1.54) million pairs of blue petrels and 99,000 (65,000–134,000) pairs of common diving petrels on the
archipelago. These are similar figures to those from the only previous estimate, made in 1980. We found breeding sooty shearwaters
for the first time, and estimated a population of several thousand pairs. We emphasise the facility of distance sampling as
an unbiased technique with practical advantages over commonly used area search methods for monitoring populations of burrowing
seabirds. These advantages include increased survey efficiency allowing a larger sample size for a given effort and a correspondingly
tighter estimation of density. |
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